Molded case circuit breakers are well known in the art. The principal components of a molded case circuit breaker are a movable upper contact arm and a movable or stationary lower contact arm. When the circuit breaker is closed, the upper contact arm is in electrical contact with the lower contact arm to allow current to flow through the circuit breaker. The operating mechanism of a typical circuit breaker is designed to protect an electrical system, coupled to the circuit breaker from high level overcurrent conditions, requiring high speed tripping and longer duration low-level overload conditions by separating the upper contact arm from the lower contact arm to open the circuit breaker. Typically, the contact arms of a molded case circuit breaker are opened and closed by an overcenter toggle mechanism. When the contacts of the circuit breaker are closed, this toggle mechanism is latched such that when the latch is released the toggle mechanism causes the upper contact arm to quickly move away from the lower contact arm, opening the circuit breaker.
The latch of a typical toggle mechanism is positioned adjacent to a trip bar which, when engaged, opens the latch, causing the electrical contact arms to open. The trip bar is typically engaged by apparatus such as a bimetallic element which bends in response to an overcurrent condition and, in so bending, causes the trip bar to rotate and release the latch. Prior art circuit breakers have also included an armature which is responsive to the magnetic field generated by current flowing through the breaker to engage the trip bar.
The overcenter toggle mechanism and the trip mechanism are relatively complex structures. As the features provided in molded case circuit breakers increase, the complexity of the operating mechanisms also tends to increase. This increasing complexity may make it difficult to produce a circuit breaker which opens consistently responsive to the same force applied to the trip bar. This inconsistency in the operation of the circuit breaker results from uncertainty in the position of the various components of the operating mechanism and trip mechanism. Because of this inconsistency in the operation of the circuit breaker, it may be difficult to produce a molded case circuit breaker which accurately conforms to a predetermined rating or which is difficult to calibrate.